The present invention relates to the high power x-ray tube arts. It finds particular application in conjunction with x-ray tubes for CT scanners and will be described with particular reference thereto. It is appreciated, however, that the invention will also find application in conjunction with other types of high power vacuum tubes.
In early x-ray tubes, electrons from a cathode filament were drawn at a high voltage to a stationary target anode. The impact of the electrons caused the generation of x-rays as well as significant thermal energy. As higher power x-ray tubes were developed, the thermal energy became so large that extended use damaged the anode.
Today, one of the principal ways to distribute the thermal loading and reduce anode damage is to rotate an anode. The electron stream is focused near a peripheral edge of the anode disk. As the anode disk rotates, the focal spot or area on the anode disk where x-rays are generated moves along an annular path or footprint. Each spot along the annular path is heated to a very high temperature as it passes under the electron stream and cools as it rotates around before returning for the generation of additional x-rays. However, if the path of travel around the anode is too short, i.e. the anode diameter is too small, or the exposure time is too long, the target area on the anode can still contain sufficient thermal energy that the additional thermal energy from again passing under the electron stream causes thermal damage to the anode surface. Because the anode is in a vacuum, dissipation of heat is retarded and thermal energy stored in the anode tends to build with each rotation of the anode. With the advent of volume CT scans, longer exposure times are becoming more prevalent.
A volume CT scan is typically generated by rotating an x-ray tube around an examination area while a couch moves a subject through the examination area. Presently, greater scan volumes at higher powers are increasingly valuable diagnostically. This diagnostic pressure has, over time, resulted in anodes of progressively larger diameter and mass which provide a longer focal spot path and allow the anode more time to dissipate the additional heat energy. Unfortunately, increasing the length of the focal spot path by increasing the diameter of a single anode requires physically larger x-ray tubes. These bigger tubes have more mass and require more space and peripheral cooling equipment in the already cramped gantry.
It is known to collimate x-rays from a single focal spot into two or more planes of radiation. One drawback of this technique is that the planes are not parallel. Further, only a small number of planes are generated. Several revolutions are needed to traverse a diagnostically significant volume.
Large diameter fixed anode x-ray tubes have been designed with multiple focal spots paths. Multiple slices are obtained sequentially by electrostatically driving an electron stream produced by a single electron gun onto, and around, a series of stationary target anode rings. The anodes are very large, on the order of a meter or more which requires elaborate vacuum constructions. Because the x-ray beams are produced sequentially only a single slice is generated at a time.
Still other systems have been proposed which use a plurality of x-ray tubes within a common CT gantry.
In another approach, a plurality of focal spots are generated concurrently on a single rotating anode. The resultant x-rays are collimated into plural parallel beams. However, multiple concurrent focal spots on a common anode multiply the thermal loading problems. See U.S. Pat. No. 5,335,255 to Seppi, et al.
In another volume imaging technique, the x-rays are collimated into a cone beam. A two dimensional detector grid detects the x-rays to provide attenuation data for reconstruction into a volume image representation. However, x-ray scatter and reconstruction artifacts are problematic with cone beam geometry.
Thus, a simpler and/or better method and system capable of generating a volume scan quickly would be useful. A quickly performed scan correspondingly decreases the amount of thermal energy absorbed by the anodes which may desirably reduce anode size. The present invention contemplates a new, improved x-ray tube assembly and method of x-ray generation which overcomes the above difficulties and others.